1. Field of the Invention
The present invention relates to a positioner for use in remotely controlling a variety of scattered devices in an industrial processing apparatus, an air-conditioning apparatus, or the like.
2. Description of the Prior Art
In remotely controlling a plurality of scattered devices, it is normal practice to provide a control device for issuing an analog signal, e.g., a signal varying in the range of from 4 to 20 mA, to as many receivers or positioners as the number of devices to be controlled for receiving the signal to control the devices, e.g., controlling the opening of regulating valves, dampers, or the like dependent on the value of a current. In some cases, two positioners are connected in series to one transmission path. However, since the device to be controlled usually has a load impedance of about 600 ohms and each of the positioners usually has an input impedance of about 250 ohms, it is impractical to connect three or more positioners to the same transmission path. Each positioner effects a controlling arithmetic operation on a preset value indicated by the analog current value and an actual feedback value detected from the device as it responds to the positioner output and performs closed-loop control in order to bring the actual feedback value into conformity with the preset value. When the controlled condition of the device becomes unstable due to the relationship between the time constant of the device as it responds and the time constant of the control loop, the positioner effects an open-loop control on the device. As the number of devices to be controlled is increased, more positioners are required to be employed for association with the respective devices, resulting in an increased cost and installation space as well as an increased number of maintenance and inspection steps. Further, the switching, as required, between the closed loop control and the open loop control requires a switching circuit arrangement, the operation of which is tedious and time-consuming.